Optical head

ABSTRACT

An optical head having a semiconductor laser and a generation of a plurality of reflected light beams from an optical disk which have polarities of intensity distribution variations which are substantially inverted to each other when a periodic structure of the optical disk crosses at least one focused spot on the disk. An optical detection system splits the plurality of reflected light beams and detects the split reflected light beams. An electrical circuit provides a focus error signal of the at least one focused spot and a tracking error signal from the plurality of reflected light beams. The electrical circuit adds focus error signals provides a difference signal between the focus error signals, amplifies tracking error signals with a gain, and obtains a difference between amplified tracking error.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation of U.S. application Ser. No. 10/131,225, filedApr. 25, 2002, now U.S. Pat. No. 6,728,173, which is a cotinuation ofU.S. application Ser. No. 09/249,290, filed Feb. 12, 1999, now U.S. Pat.No. 6,400,664, the subject matter of which is incorporated by referenceherein.

BACKGROUND OF THE INVENTION

The present invention belongs to an optical head used in an optical diskdevice, and more particularly relates to a technique for enhancing aperformance in detection of a position controlling signal for an opticalspot thereof.

Conventional techniques on methods for controlling a focal pointposition in an optical disk device are described in, for example,“Fundamentals and Applications of Optical Disk Storage”, Y. Tsunoda,1995, 1st edition (Korona Corp., Tokyo), pp. 79-83. According to thisliterature, there are the following methods: Foucault method (Knife edgemethod), an astigmatic method, a beam size detection method, an imagerotating method, and so on. From criteria such as simplicity of anoptical system required, the ease with which the adjustment can be made,and the ease with which combination with a tracking detection can beachieved, the most prevailing method, at the present stage, is theastigmatic method. In the astigmatic method, however, there existed aproblem that, when an optical spot crosses a track on the surface of astorage film, a disturbance is apt to occur in a focus error signal inassociation with a decentering of an optical disk. This disturbance islikely to occur especially when astigmatism takes place in a focusedspot or the optical spot is shifted on an optical detector. Examples ofmethods for reducing the disturbance are disclosed as follows: A methodof reducing the disturbance by blocking light out of a central portionof a detected light beam is disclosed in JP-A-6-162527 andJP-A-6-309687, a method of reducing it by adjusting rotation of anobjective lens is disclosed in JP-B-5-68774, and a method of reducing itby means of an operation between a light with astigmatism and a lightwithout astigmatism in a detected system is disclosed in JP-A-5-197980.None of them, however, is a fundamental method for solving theabove-described problem. Thus, at the present stage, the reducing effectobtained is not necessarily enough.

In particular, in a land-groove type optical disk employed in a DVD-RAMplanned to be brought into a commercial stage soon, the disturbanceoccurs quite outstandingly. The reason is as follows: In the land-groovetype optical disk, a width of a guiding groove (groove) is substantiallyequal to a width of a portion of a guiding inter-groove (land), andinformation is stored on the both sides thereof. On account of this, apitch of the guiding groove itself, when compared with an optical spot,is formed to be larger than in conventional optical disks. Thisextraordinarily intensifies a tracking error signal according to apush-pull method described later, thus causing the disturbance to occurquite outstandingly. This condition, accordingly, brings about asituation that, in an optical head for the DVD-RAM, it can not be helpedemploying the Foucault method or the beam size detection method theconfiguration or the adjustment of which is complicated.

Conventional techniques on methods for controlling a tracking in anoptical disk device are similarly described in, for example, theabove-cited “Fundamentals and Applications of Optical Disk Storage”, Y.Tsunoda, 1995, 1st edition (Korona Corp., Tokyo), pp. 83-92. Accordingto this literature, there are methods such as a three-spots method and adiffracted light differential method (push-pull method). Judging fromcriteria such as simplicity of an optical system required, the ease withwhich the adjustment can be made, and a resistance to the disturbance,the three-spots method is mainly employed in a read only type opticaldisk such as a compact disk (CD). Meanwhile, the push-pull method ismainly employed in the case of a magneto-optical disk or the DVD-RAMwhich needs a high laser emission power at the time of the recording. Atthis time, there can be considered another way in which, exchanging theroles with each other, the push-pull method is employed toward the CDand the three-spots method is employed toward writable optical disks.However, there exist circumstances which make such an employmentimpossible.

In performing a CD pick up, in order to cause a focused spot to follow adecentering of the optical disk for the necessity of low price, theobjective lens is moved by only being mounted on a lens actuator. Then,if the push-pull method is employed, it turns out that a detected lightbeam moves on the optical detector. This phenomenon appears as anoff-set. Also, at a pit depth of λ/(4n) (λ: light wavelength, n:substrate refractive index) at which a signal amplitude becomes largestin the reproduction-only type optical disk, there are the followingproblems: Of diffracted light by means of a periodic structure of trainof pits in the radial direction, 0th order light becomes smaller. Inaddition to this, even when the focused spot is off-track, no unbalanceoccurs in interference intensity between the 0th order light and ±1storder diffracted lights. This makes it impossible to obtain the trackingerror signal.

Meanwhile, in the storage-able optical disks, especially in themagneto-optical disk, compensation for the decentering of the opticaldisk is usually performed by an actuator called a coarse actuator. Thecoarse actuator mounts the optical head or only a portion of theobjective lens and an objective lens actuator so as to allow the opticalspot to come near to a proximity of a track to be objected. Namely, themagneto-optical disk is constituted in such a manner that, of a trackingerror, the low frequency components are compensated by the coarseactuator and the high frequency components are compensated by theobjective lens actuator, thereby enhancing a reliability needed for thestorage operation. Accordingly, an amount of movement by the objectivelens actuator is lower than in the read only type optical disk such asthe CD. This makes it possible to employ the push-pull method which hashigher light utilization efficiency than the three-spots method does.

Also, if the three-spots method is employed toward the storage-ableoptical disks, as described on page 127 of “Technical Digest ofSymposium on Optical Memory '86”, there take place the followingproblems: First, in an optical disk such as the DVD-RAM, i.e. the typeof optical disk that performs the storage by means of a variation inreflectance of a storage mark, at the time of the storage operation,there arises a difference in the amount of light between a precedingsub-spot and a subsequent sub-spot. This causes an off-set to occur inthe tracking error signal. Also, in the case of the magneto-opticaldisk, there exists a feedback light back to a semiconductor laser. Onaccount of this, a tilt of the disk unbalances a condition ofstray-lights interference on the both sides of sub-spots. This alsocauses an off-set to occur. Moreover, as described already, theland-groove type optical disk is employed in the DVD-RAM. Thiscircumstance can also be mentioned as a reason for making it impossibleto employ the three-spots method toward the DVD-RAM. Namely, in theland-groove type optical disk, a width of the land portion is originallymade equal to that of the groove portion in order to make an amount ofreflected light of the land portion equal to that of the groove portion.This necessarily results in a fact that, even when an optical spot isoff-track, the amount of light scarcely varies, thus making itimpossible to obtain a tracking error signal according to thethree-spots method. Accordingly, it can not be helped employing thepush-pull method in the DVD-RAM. However, unlike the case of themagneto-optical disk, it is required to lower the price of the DVD-RAMdown to a price close to the price of the CD. Consequently, it becomesabsolutely necessary to reduce the off-set in a tracking error signalwhich accompanies the movement of the objective lens according to thepush-pull method.

A conventional technique for solving the above-mentioned problem in theDVD-RAM is described in, for example, “National Technical Report”, Vol.40, No. 6, (1994), pp. 771-778. Here, the optical disk device isconstituted as follows: The objective lens, a λ/4 plate, and apolarizing diffraction grating are integrally mounted on an objectivelens actuator. Moreover, the polarizing diffraction grating isconstituted so that interference regions, in which, of diffracted lightby mean of the disk, +1st order diffracted light and −1st orderdiffracted light each interfere with 0th order light, are diffractedwith a different diffraction angle, respectively. This constitutionmakes it possible to separate, on the optical detector, the interferenceregion between the +1st order diffracted light and the 0th order lightfrom the interference region between the −1st order diffracted light andthe 0th order light. From this, the above-mentioned literature shows thefollowing: If a dual-divided optical detector is constituted so that,when the objective lens is moved, the lights do not stray out of theoptical detector, it becomes possible to eliminate the off-set caused bythe phenomenon that the optical spots move on the optical detector.

Also, employing the polarizing diffraction grating as a diffractiongrating makes the following possible: When a light heading for the diskpasses through the polarizing diffraction grating, the diffractionefficiency is made substantially equal to zero, and when a reflectedlight from the disk passes through the polarizing diffraction gratingagain, the diffraction efficiency is caused to become an appropriatevalue. Meanwhile, in the case of a non-polarizing ordinary diffractiongrating, it diffracts the light heading for the disk, too, thus makingit impossible to avoid a loss of the amount of light. Employing thepolarizing diffraction grating in this way allows only the necessarydiffraction of the reflected light to occur, thus making it possible toprevent the loss of the amount of light.

However, in this conventional example, the objective lens, the λ/4plate, and the polarizing diffraction grating are integrally mounted onthe objective lens actuator. This constitution results in a problem thata movable portion of the actuator becomes heavy, thus restricting aresponse speed of the actuator down to a low level. Since optical disksare being improved in the storage density and at the same time arebecoming faster in the transfer rate year by year, the above-describedconventional example is not able to meet the trend of even furtherspeeding-up in the near future.

Another method, which, with no other optical component except theobjective lens mounted on the objective lens actuator, makes it possibleto eliminate the tracking error signal off-set which accompanies themovement of the objective lens according to the push-pull method, isdisclosed in the above-described “Technical Digest of Symposium onOptical Memory '86”, pp. 127-132. This method is referred to as adifferential push-pull method. In the method, the three-spots method isemployed, and respective tracking error signals according to thepush-pull method are subtracted on a main-spot and two sub-spots,thereby eliminating the tracking error signal off-set which accompaniesthe movement of the objective lens. Namely, in the method, the sub-spotsare located in such a manner that they are shifted on the both sides ofthe main spot by one-half of a period of the guiding groove, therebysimultaneously detecting a light beam in which variations ininterference intensity distribution of a reflected light beam reflectedfrom the disk in association with an off-track are inverted, and thusgenerating opposite phase tracking error signals which contain theoff-set in the same phase. Then, these opposite phase tracking errorsignals are subtracted, thereby allowing only the off-set to becancelled. According to this conventional example, the ratio of the gainto amplify the signal by the main spot to the gain of the signal by thesub-spot is chosen so as to compensate the intensity unbalance caused bydiffraction efficiency characteristics of the diffraction grating togenerate sub-spots. The use of this conventional example, with no otheroptical component except the objective lens mounted on the objectivelens actuator, basically makes it possible to eliminate the trackingerror signal off-set which accompanies the movement of the objectivelens according to the push-pull method. In the present conventionalexample, however, no countermeasure is taken against the mixture of thedisturbance into the focus error signal when a focused spot crosses theguiding groove in the astigmatic focus error detecting method describedearlier. Also, as described in the present conventional example, whenone of the sub-spots lies in a post-stored track and the other lies in apre-stored track, the effect of reducing the off-set is not enough.Further, although not described in the present conventional example,when a total amount of reflected light on the guiding grooves differsfrom a total amount of reflected light on the guiding inter-grooves, theoff-set also remains in the present conventional example. This situationarises when a width of the guiding groove is not equal to that of theguiding inter-groove. However, in the case of the DVD-RAM employing theland-groove type optical disk in which the width of the guiding grooveis equal to that of the guiding inter-groove, such a situation alsoarises if the main-spot lies in the post-stored track and the twosub-spots lies in the pre-stored track or in the case opposite thereto.Still further, in the present conventional example, there exist theplurality of optical spots. This brings about a disadvantage in thelight utilization efficiency at the time of the storage.

Moreover, the gains to amplify the signals by main spot and sub-spotschosen in this conventional method is insufficient to cancel the effectcompletely. The reason is as follows. As described later, when a widthof the guiding groove does not substantially equal to half of the pitchof guiding grooves, the reflectance of the light when the focused spotis at the guiding groove is different from that when the focused spot isat the inter-groove. It is also necessary to compensate this unbalanceof reflectance for perfect offset cancellation. For the higher thedensity of the optical disk, the allowance of the offset is the severer.Therefore this insufficient cancellation must be a problem, recently.

Still more, in this conventional example, the optical disk was not aland-groove type optical disk. Therefore, there is no cross-talk fromstored information signal, because the sub-spots on the optical disk arenot on the information track at readout process. In the case ofland-groove type optical disk such as DVD-RAM, however, the sub-spots isalso on the information track of reading out from the disk. This resultsin disturbance to the tracking error signal.

Another method, which cancels the disturbance in the focus error signalof astigmatic method, is disclosed in the JP-A-4-168631. Also in thismethod, the main spot and sub-spots by a diffraction grating ispositioned onto the optical disk at the distances of the half of thepitch of guiding grooves in the radial direction of the disk. Thereflected beams from these focused spots pass through a cylindricallens, then detected by three quadratic photo-detectors, respectively.From the output signal of these photo detectors, three focus errorsignals are obtained by calculation in the electric circuit. These focuserror signals are amplified with gains which are proportional to thereciprocals of the light intensity of each focused spot on the opticaldisk, which are not proportional to the reciprocal of the reflectedlight intensity. Then summation of these amplified focus error signalsis calculated in the electric circuit. Employing this method, the extradisturbance to the conventional focus error signal by aberrations ormiss-alignment of the optical components or photo detector can beeliminated. The optimum gains for disturbance cancellation for thismethod is different from those for differential push-pull method asmentioned. However, in this method, no tracking method is disclosed.Further more, if the differential push-pull method described in theconventional literature itself is employed with this focusing errordetection method, it is necessary to set the gains to amplify the signalby each reflected light beam equal between in the focus error signal andin the tracking error signal, namely proportional to the reciprocals ofthe light intensity of incident focused spots on to the optical disk. Itresults in the insufficient cancellation of the offset of tracking errorsignal as mentioned.

SUMMARY OF THE INVENTION

In view of the above-described conventional techniques, in the methodand the device for detecting the focal point shift, a problem to besolved by the present invention is to fundamentally eliminate thedisturbance which occurs in the focus error signal in association withthe decentering of an optical disk when an optical spot crosses a trackon the surface of the storage film.

Also, another problem to be solved by the present invention is tofundamentally cancel the off-set which occurs simultaneously in thetracking error signal in association with the movement of the lens.

Also, when employing a method such as the differential push-pull methodin which a light beam, in which variations in interference intensitydistribution of a reflected light beam at the time when an optical spoton the disk crosses the guiding groove are inverted, is generatedsimultaneously with the ordinary light beam and thus the opposite phasetracking error signals which contain off-set components with the samephase are generated so as to cause the same phase off-set to becancelled, another problem to be solved by the present invention is tocancel an off-set which occurs from a difference in the total amount ofreflected light between these light beams.

Also, another problem to be solved by the present invention is not onlyto cancel, in the differential push-pull method, the off-set whichoccurs in the tracking error signal in association with the movement ofthe lens but also to fundamentally eliminate, in the focal point shiftdetecting method, the disturbance which occurs in the focus error signalin association with the decentering of an optical disk when an opticalspot crosses a track on the surface of the storage film.

Also, another problem to be solved by the present invention is toobtain, with the sub-spots in the differential push-pull method locatedon the same track as the main-spot, the same effect of canceling thetracking error signal off-set which accompanies the movement of theobjective lens.

Also, another problem to be solved by the present invention is toconstitute the optical disk device so that a single spot on the diskexhibits the same effect as the differential push-pull method does.

Also, another problem to be solved by the present invention is to obtainthese effects toward the astigmatic focal point shift detecting methodand the push-pull tracking detecting method in particular.

Also, another problem to be solved by the present invention is toillustrate a configuration of an optical detector which allows theseeffects to be obtained.

Also, another problem to be solved by the present invention is toenhance performance in the canceling of the tracking error signaloff-set due to the movement of the objective lens when combining thedifferential push-pull method with the additive astigmatic method.

Also, another problem to be solved by the present invention is toeliminate an influence of the disturbance due to the information pitswhen combining the differential push-pull method with the additiveastigmatic method so as to apply them together to the land-groove typeoptical disk.

In order to solve the above-described problems, an optical headcomprises at least a semiconductor laser, a light-converging opticalsystem for converging an emitted light from the semiconductor laser ontoan optical disk having a periodic structure in a radial direction as atleast one focused spot, an optical detection system for detecting areflected light from the optical disk, and an electrical circuit forcalculating an amount of received light through a photoelectricconversion thereof so as to obtain at least one of a focus error signalof the focused spot converged on the optical disk, a tracking errorsignal of the focused spot converged on the optical disk, and a datasignal stored in the optical disk. The light-converging optical systemincludes means for generating a plurality of reflected light beams inwhich polarities of their intensity distribution variations at the timewhen the periodic structure crosses the focused spot on the optical diskare substantially inverted with each other, the optical detection systemincludes means for splitting and simultaneously detecting the pluralityof reflected lights, and the electrical circuit obtains the focus errorsignal by taking summation of focus error signals of the respectivereflected light beams so that variations of the focus error signalcaused by their intensity distribution variations cancel out with eachother.

Also, at this time, a difference between respective tracking errorsignals of the plurality of reflected lights the polarities of which areinverted with each other is simultaneously defined as the tracking errorsignal.

Moreover, at this time, in defining, as the tracking error signal, thedifference between the respective tracking error signals of theplurality of reflected light beams the polarities of which are invertedwith each other, in the electrical circuit, the respective trackingerror signals are amplified with a gain which is proportional to a ratiobetween reciprocals of respective total amounts of the reflected lightswhen one of said focused spot is at the information track of saidoptical disk, and after that a difference between the respectivetracking error signals thus amplified is calculated, then being definedas the tracking error signal.

Also, in these constitutions, there is provided a beam splitting devicefor splitting the reflected light beam from the optical disk off from anoptical path extending from the semiconductor laser, and the means forgenerating said plurality of reflected light beams the polarities ofintensity distribution variations of which are substantially invertedwith each other is a diffraction grating located between thesemiconductor laser and the beam splitting device. Moreover, thediffraction grating is installed in such a manner as to be tilted towardthe radial direction of the optical disk so that focused spots of ±1storder diffracted lights by means of the diffraction grating are locatedin such a manner that, on the optical disk and with reference to afocused spot of a 0th order light, they are shifted by about one-half ofa period of the above-described periodic structure in oppositedirections to each other in the radial direction.

Also, there is provided a beam splitting device for splitting thereflected light beam from the optical disk off from an optical pathextending from the semiconductor laser, and the means for generating theplurality of reflected light beams the polarities of intensitydistribution variations of which are substantially inverted with eachother is a diffraction grating located between the semiconductor laserand the beam splitting device. Moreover, the diffraction grating hasgratings the phase of which is inverted at an interval of substantiallyλD/(2NA·P) (λ: light wavelength, NA: numerical aperture of an objectivelens, P: period of the periodic structure in the radial direction on theoptical disk, D: effective light beam diameter on the diffractiongrating) in regions of a common width in the radial direction on theoptical disk. The diffraction grating is installed in such a manner thata direction of the gratings thereof is in parallel to a tangentialdirection of the optical disk so that, on the optical disk, focusedspots of ±1st order diffracted lights by means of the diffractiongrating are located on the same track as a focused spot of a 0th orderlight. Furthermore, the optical detection system splits and detectsthese focused spots. Then, a data signal is obtained from an amount ofreceived light signal resulting from the 0th order light.

Still further, there is provided a beam splitting device for splittingthe reflected light beam from the optical disk off from an optical pathextending from the semiconductor laser, and the means for generating theplurality of reflected light beams the polarities of intensitydistribution variations of which are substantially inverted with eachother is a polarizing phase shifter located between the semiconductorlaser and the beam splitting device. The polarizing phase shifter isconstituted so that it relatively inverts a phase of a linearlypolarized light component, which is polarized in a specific direction,at an interval of substantially λD/(2NA·P) (λ: light wavelength, NA:numerical aperture of an objective lens, P: period of the periodicstructure on the optical disk, D: effective light beam diameter on adiffraction grating) in regions of a common width in the radialdirection on the optical disk, and a phase of a linearly polarized lightcomponent perpendicular to the linearly polarized light component is notvaried over a whole system of the polarizing phase shifter. Furthermore,the optical detection system splits and detects these polarized lightcomponents with the use of a polarizing beam splitting device. Then, adata signal is obtained from the polarized light component to which nophase inversion is added.

In particular, the above-described constitutions are embodied byemploying the astigmatic method for the focus error detection and byemploying the push-pull method for the tracking error detection.

Also, in the optical detection system, there is provided an opticaldetector in which there exist at least two sets of optical detectionregions each of which receives a single optical spot with a four-dividedoptical detection region.

Also, an optical head includes a semiconductor laser, a light-focusingoptical system for focusing, as at least one focused spot, an emittedlight from the semiconductor laser onto an optical disk which has aperiodic structure such as guiding grooves in its radial direction, anoptical detection system for detecting a reflected light from theoptical disk, and an electrical circuit for obtaining, from thereflected light, both a focus error signal of one of the focused spotsand a tracking error signal thereof. In the optical head, sub-spots, forexample, are located by an apparatus such as a diffraction grating insuch a manner that they are shifted from a main-spot by one-half of aperiod of the guiding grooves, thereby generating two kinds of and, foreach of the kinds, at least one or more of reflected light beams inwhich polarities of their intensity distribution variations at the timewhen the periodic structure crosses the focused spots are substantiallyinverted with each other. The optical detection system splits anddetects the plurality of reflected light beams. In the electricalcircuit, focus error signals, which are obtained by each adding focuserror signals generated from the two kinds of and, for each of thekinds, at least one of reflected light beams, are amplified and addedfurther, thereby obtaining the focus error signal. Moreover, trackingerror signals, which are obtained by each adding tracking error signalsgenerated from the two kinds of and, for each of the kinds, at least oneof reflected light beams, are amplified and subtracted from each other,thereby obtaining the tracking error signal. At this time, an opticaldisk such as the land-groove type optical disk is used in which theguiding grooves constitute the periodic structure and, as compared withan occasion when one of the focused spots is situated at a guidinggroove, an error of the reflectance on an occasion when it is situatedat a guiding inter-groove falls within a range of ±10% thereof. The useof such type of optical disk makes it possible to cause an amplificationgain ratio between the tracking error signals of the two kinds ofreflected light beams to coincide with an amplification gain ratiobetween the focus error signals of the two kinds of reflected lightbeams.

Also, in a similar optical head, in a case where the optical disk usedis an optical disk other than the land-groove type optical disk, i.e.,in a case where the reflectances differ between an occasion when one ofthe focused spots is positioned on an information track of the opticaldisk and an occasion when it is positioned at a position which is apartfrom the information track by one-half of the period of the periodicstructure, it turns out that the amplification gain ratio between thetracking error signals of the plurality of reflected light beams differsfrom the amplification gain ratio between the focus error signals of theplurality of reflected light beams.

Also, in the optical head, the electrical circuit for detecting thesub-spots is provided with a frequency characteristic which makes itpossible to cut off a frequency bandwidth of a read-out signal ofrecorded information written in the optical disk.

Other objects, features and advantages of the present invention willbecome apparent from the following detailed description of theembodiments of the invention taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram for showing a constitution of an optical system in abasic embodiment of the present invention;

FIG. 2 is a diagram for showing locations of focused spots on an opticaldisk and intensity distributions of reflected light beams at that time;

FIG. 3 is a diagram for showing a circuit calculation method of anoutput of a detector;

FIG. 4 is a diagram for showing a calculation method of the output ofthe detector;

FIG. 5 is a diagram for explaining a tracking error signal off-set dueto movement of an objective lens;

FIG. 6 is a diagram for explaining an disturbance into a focus errorsignal due to astigmatism;

FIG. 7 is a diagram for showing a constitution of an optical system inan embodiment in which a phase-inverted diffraction grating is employed;

FIG. 8 is a diagram for explaining a detailed structure of thephase-inverted diffraction grating;

FIG. 9 is a diagram for explaining a manner in which phase shift regionsare overlapped in diffraction of a phase-inverted light by means ofoptical disk guiding grooves;

FIG. 10 is a diagram for showing interference phase differences added toan optical disk diffracted light by means of the phase-inverteddiffraction grating;

FIG. 11 is a diagram for explaining a manner in which, when the movementof the objective lens exists, phase shift regions are overlapped indiffraction of a phase-inverted light by means of optical disk guidinggrooves;

FIG. 12 is a diagram for showing a constitution of an optical system inan embodiment of the present invention in which a polarizing phaseshifter is employed;

FIG. 13 is a diagram for explaining the principle of the polarizingphase shifter;

FIG. 14 is a diagram for explaining calculation of an disturbance due toan ordinary crossing over a guiding groove by means of a focus errorsignal;

FIG. 15 is a diagram for explaining calculation of an disturbance due toa crossing over a guiding groove by means of a focus error signal at thetime of employing the phase-inverted diffraction grating;

FIG. 16 is a diagram for explaining calculation of an disturbance due toa crossing over a guiding groove by means of a focus error signal in adifferential push-pull method;

FIG. 17 is a diagram for explaining lens shift characteristics of atracking error signal with the use of an ordinary detecting light beamfor a focus error signal;

FIG. 18 is a diagram for explaining lens shift characteristics of atracking error signal with the use of a detecting light beam for a focuserror signal at the time of employing the phase-inverted diffractiongrating;

FIG. 19 is a diagram for explaining lens shift characteristics of atracking error signal with the use of a detecting light beam for a focuserror signal in the differential push-pull method;

FIG. 20 is a diagram for showing an embodiment of an optical systemconstitution according to the present invention in which thereproduction is possible in a DVD, a DVD-RAM, a CD, and a CD-R;

FIG. 21 is a diagram for explaining details of a constitution for thedetection in the embodiment in FIG. 20;

FIG. 22 is a diagram showing a modified example of the circuitcalculation method of the detector output in FIG. 3; and

FIG. 23 is a characteristic diagram representing a frequencycharacteristic of a gain of an amplifier and a read-out signal intensityof the detector.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The description will be given below concerning embodiments of thepresent invention, using the accompanying drawings.

FIG. 1 is a diagram for showing a constitution of an optical system in abasic embodiment of the present invention. A light beam emitted from asemiconductor laser 101 produces a diffracted light by passing through adiffraction grating 102. The diffracted light, by way of a beam splitter103, a triangle reflection prism 104 and an objective lens 106, forms amain-spot 108 of a 0th order light and two sub-spots 109, 110 of ±1storder diffracted lights on an optical disk 107. A reflected light beam,by way of the objective lens 106 and the triangle reflection prism 104again, is reflected at the beam splitter 103. Then, the reflected lightbeam is provided with an astigmatism for detecting a focal point shiftby a cylindrical lens 111, thus being received by an optical detector115. The optical detector 115 is separated into a 0th order lightfour-divided optical detection region 112 and ±1st order diffractedlights four-divided optical detection regions 113, 114. The two kinds ofoptical detection regions are independent of each other in performingthe detection. Here, the diffraction grating 102 is located in such amanner as to be tilted to some extent so that the ±1st order diffractedlights on the optical disk are located in such a manner as to be shiftedon the both sides of the 0th order light by one-half of a guiding groovepitch.

FIG. 2 is a diagram for showing locations of optical spots on theoptical disk and intensity distributions of reflected light beams atthat time. FIG. 2 shows a case in which a 0th order light 201 and ±1storder diffracted lights 202, 203 are shifted slightly on the left sidewith reference to groove portions 204 and land portions 205, a case inwhich they are just on-track, and a case in which they are shiftedslightly on the right side. At this time, intensity variations of a 0thorder light-detected light beam 206 and a ±1st order diffractedlights-detected light beam 207, as illustrated in FIG. 2, are shifted indirections opposite to the directions of the above-mentionedtrack-shifts of the optical spots on the disk. This is because the ±1storder diffracted lights 202, 203 on the disk are located in such amanner that they are sifted with reference to the 0th order light 201 byone-half of the track pitch. There occur such light intensitydistributions of the detected light beams in correspondence withpositions of the optical spots on the optical disk. As described in, forexample, the literatures cited earlier, this knowledge itself has beenknown publicly.

FIG. 3 is a diagram for showing a circuit calculation method of outputsof the optical detector. Incidentally, on the optical detector 115, theintensity distributions of the reflected light beams are rotated by 90degrees because of the astigmatism for detecting a focal point shift orfocus error. Here, a focus error signal (AF signal) is obtained byadding components in the same diagonal direction of correspondingdivided outputs of the divided optical detector 112 for the main-spotand the divided optical detectors 113, 114 for the sub-spots, and thenby calculating the differential signal thereof with the use of adifferential amplifier 303. Calculating the focus error signal in thisway allows only the disturbance to be canceled out because, when thefocused spots cross a guiding groove on the disk, variations inintensity distribution of the sub-spots are inverted with a variation inintensity distribution of the main-spot. At this time, usually, anamount of light of the sub-spots is made smaller than that of themain-spot. Accordingly, the calculation is performed after signaloutputs of the sub-spots are amplified by an amplifier 301 by an amountcorresponding to the ratio between the amount of light of the sub-spotsand that of the main-spot. In this embodiment, however, there exist thetwo sub-spots. Assuming an intensity of the main-spot as A and that ofthe sub-spot as B, the gain on each sub-spot, actually may take a valueobtained by multiplying, by A/(2B), amplification gains of the signalsby the two sub-spots with reference to the main-spot. Meanwhile, atracking error signal (TR signal) is obtained by first adding,alternately between in the main-spot and in the sub-spots, an output forevery two regions divided into the right and left in FIG. 3, and then bycalculating the differential output thereof with the use of adifferential amplifier 304. Calculating the tracking error signal inthis way makes it possible to obtain a tracking error signal in whichonly the off-set components are canceled out, because, when the focusedspots cross the guiding groove on the disk, the variations in intensitydistribution of the sub-spots are inverted with the variation inintensity distribution of the main-spot and in addition the off-set dueto the lens shift is not inverted. Here, from the above-describedoriginal location, when the main-spot is situated on a land portion, thesub-spots are situated on groove portions. This, when a width of theland portion is different from that of the groove portion, results in adifference in the amount of reflected light between the main-spot andthe sub-spots, thereby making the off-set canceling insufficient. Insuch a case, signals of the sub-spots are amplified by an amplifier 302so that the difference in the amount of reflected light therebetween iscompensated. For example, when the information tracks exist on the landportions, the again of the amplifier 302 of the sub-spots, may take a/bwhere a represents a reflectance of the land portion as and b representsthat of the groove portion. Also, in some cases, the output of themain-spot may be lower. In such a case, the main-spot, on the contrary,is amplified. Otherwise, the gain of the amplifier 302 is made equal to1 or less. The above-described calculation method makes it possible tosimultaneously obtain the tracking error signal without the off-set dueto the lens shift and the focus error signal without the disturbance atthe time of crossing the guiding groove. In the mean time, concerning areproducing signal, a total amount of light of the main-spot isoutputted using a differential amplifier 305. Incidentally, here, theoptical disk employed is assumed to be an optical disk such as areproduction-only type optical disk or a phase-varied type optical diskwhich allows a signal to be reproduced with the use of the amount ofreflected light. However, in the case of the magneto-optical disk aswell, there exists no other difference except a difference which resultsfrom defining the data signal as a differential signal between twosignal outputs in which the polarized light components are split.Consequently, it is possible to detect the focus error signal and thetracking error signal with the use of the present embodiment.

FIG. 4 is a diagram for summarizing the calculation method at this time.As a conclusion, what should be done is just to perform the calculationsas illustrated in FIG. 4 toward four outputs a, b, c, d of the 0th orderlight four-divided optical detection region 112 and respective fouroutputs e, f, g, h, i, j, k, l of the ±1st order diffracted lightsfour-divided optical detection regions 113, 114. Incidentally, here, areference note RF denotes a data signal, AF a focal point shift errorsignal, and TR a tracking error signal.

The above-described embodiment has generally assumed the case in whichthe reflectances differ between an occasion when a focused spot issituated at the guiding groove and an occasion when it is situated atthe guiding inter-groove. However, in the land-groove type optical diskused in the DVD-RAM disk, the width of the guiding groove issubstantially equal to one-half of the track pitch, and thus thereflectances substantially equal between the occasion when a focusedspot is at the groove portion and the one when it is at the landportion. This, by omitting the amplifier 302 in FIG. 3, makes itpossible to simplify the circuit configuration as illustrated in FIG.22. Incidentally, even in the land-groove type optical disk, because ofthe fabricating error, a difference in the reflectance in the landportion toward the groove portion can be about, at the maximum, ±10%.Concerning the difference of this extent, however, the computersimulation has demonstrated the following: When an effective diameter ofthe objective lens is set to be 4 mm, even if the lens shift is 0.4 mm,the track off-set turns out to be about 0.01 μm in a DVD-RAM disk thegroove pitch of which is 1.48 μm. This means that, in the configurationin FIG. 22 as well, the track off-set is allowable. Conversely, providedthat the allowable value of the track off-set is equal to 0.05 μm, thedifference in the reflectance in the land portion toward the grooveportion has been found to be about 1.6 times under the same conditions.This means that, in an ordinary optical disk other than the land-groovetype optical disk, this value becomes more than two times greater.Accordingly, the configuration in FIG. 22, after all, can be applicableonly to the land-groove type optical disk.

Also, in the land-groove type optical disk, there exist the informationtracks at the guiding grooves as well as at the guiding inter-grooves.Consequently, when the main-spot is situated on an information track,the sub-spots, naturally, are situated on the adjacent tracks. At thistime, there occurs a mixture of recorded information into the sub-spots,which has not been assumed except for the case of the land-groove typeoptical disk. In order to avoid this phenomenon, it is appropriate tolet an amplifier 301 in FIG. 22 have a frequency characteristic asillustrated in FIG. 23. In FIG. 23, a horizontal axis in the coordinateindicates the frequency, a vertical axis on the left side indicates thefrequency characteristic of a gain of the amplifier, and a vertical axison the right side indicates an intensity of a read-out signal oninformation tracks in a detector. Although the read-out signal lies in ahigher frequency bandwidth as compared with control signals such as thetracking error signal or the focus error signal, a signal actuallydetected by the detector is a one resulting from synthesizing thesesignals. Here, by letting the amplifier have a characteristic that thegain becomes lower in the read-out signal bandwidth, it is possible toobtain a control signal without the disturbance.

The optical disk in the embodiment mentioned upper was not specializedto land-groove type optical disk. In the case of land-groove typeoptical disk such as DVD-RAM, the reflectance of the light issubstantially equal when between the focused spot on the optical disk isin groove and in inter-groove. Therefore the amplifier 302 in the FIG. 3can be omitted as in FIG. 22. Of course, even in the land-groove disk,these reflectances have some error approximately 10%. However, it isexamined by computer simulation that this amount of deviation isallowable.

In FIG. 23, the frequency spectrum of the readout signal, servo controlsignal, and frequency transfer characteristics of the amplifiers in FIG.22 is shown. The amplifier has the frequency transfer characteristics tosubstantially cut off the frequency band of stored information signal inthe optical disk.

Next, an analytical explanation will be given below concerning thereason why such a calculation method makes it possible to cancel thetracking error signal off-set due to the movement of the objective lens.According to “Journal of Optical Society of America”, 1979, Vol. 69, No.1, pp. 4-24, distribution of a reflected light beam by means of theperiodic structure of the optical disk is obtained as follows: In scalardiffraction approximation, products of Rm, i.e. m-th order Fouriercoefficients of reflectance distribution of the optical disk, and a (x,y), i.e. complex amplitude distribution of an incident light beam, areshifted by a quantity of mNA/Pλ (NA: numerical aperture, P: period ofguiding grooves, λ: wavelength), i.e. distribution shift due to a m-thorder diffraction, and, after being multiplied by exp(i2 πmu_(o)/P),i.e. phase component based on a spot position of the main spot u_(o),are added, thus obtaining the distribution. Namely, the followingEquation (1) is obtained: $\begin{matrix}{{a^{\prime}\left( {x,y} \right)} = {\sum\limits_{m}{R_{m}{a\left( {{x - \frac{m\quad{NA}}{P\quad\lambda}},y} \right)}{\mathbb{e}}^{{{\mathbb{i}}2\pi}\frac{\quad m}{P}u_{0}}}}} & (1)\end{matrix}$

-   a(x, y): Complex amplitude distribution of incident light-   P: Period of guiding grooves-   λ: Wavelength-   NA: Numerical aperture-   m: Order of diffraction by grooves-   u₀: Spot position in radial direction

Here, Rm, which corresponds to a complex amplitude of a m-th orderdiffracted light at the time when a parallel light beam with anamplitude 1 is launched into the optical disk at an angle perpendicularthereto, is represented by Equation (2): $\begin{matrix}{R_{m} = {\frac{1}{P}{\int_{{- P}/2}^{P/2}{{R(u)}{\mathbb{e}}^{{- {\mathbb{i}2\pi}}\frac{\quad m}{P}u}\quad{\mathbb{d}u}}}}} & (2)\end{matrix}$

-   u: Radial coordinate on the disk-   R(u): Distribution of complex amplitude reflectance of disk surface,    , and, in particular, in the case of rectangular grooves with a    width w and a groove depth d normalized by the wavelength, Rm is    represented by Equation (3): $\begin{matrix}    {R_{m} - {\sin\quad c\quad m} - {\frac{w}{P}\left( {1 - {\mathbb{e}}^{{- {\mathbb{i}}}\quad 4\quad\pi\quad d}} \right)\sin\quad c\quad\frac{m\quad w}{P}}} & (3)    \end{matrix}$    -   w: Guiding groove width    -   d: Groove depth normalized by wavelength

Incidentally, here, sinc X has the relation expressed by Equation (4):$\begin{matrix}{{\sin\quad c\quad x} = \left\{ \begin{matrix}\frac{\sin\quad\pi\quad x}{\pi\quad x} & \left( {x \neq 0} \right) \\1 & \left( {x = 0} \right)\end{matrix} \right.} & (4)\end{matrix}$

Using these equations, and provided that the incident light beam has noaberration and the amplitude is uniform within an objective lens pupilsurface, interference intensities between the 0th order light and the±1st order diffracted lights by means of the periodic guiding grooves inthe optical disk are represented by Equation (5): $\begin{matrix}{{I_{0,{\pm 1}}\left( {x,y} \right)} = {{R_{0}}^{2} + {R_{\pm 1}}^{2} + {2{R_{0}}{R_{\pm 1}}{\cos\left( {\varphi \mp {\frac{2\pi}{P}u_{0}}} \right)}}}} & (5)\end{matrix}$

Incidentally, here, φ has the relation expressed by Equation (6):

 φ=arg(R _(±1))−arg(R ₀)  (6)

Using these equations, a tracking error signal TR according to apush-pull method at the time when there exists no movement of the lensis represented by Equation (7): $\begin{matrix}\begin{matrix}{{TR} = {S\left( {I_{0,{+ 1}} - I_{0,{- 1}}} \right)}} \\{= {4S{R_{0}}{R_{\pm 1}}\sin\quad{\varphi sin2}\quad\pi\quad\frac{u_{0}}{P}}} \\{= {4S\quad\frac{w}{P^{2}}\sin\quad c\quad\frac{w}{P}\sin\quad{c4}\quad\pi\quad d\quad\sin\quad 2\quad\pi\quad\frac{u_{0}}{P}}}\end{matrix} & (7)\end{matrix}$

-   S: Area size of interference region of 0th and ±1st order    diffraction

Here, as shown in FIG. 5, assuming that an optical spot 502 on adual-divided optical detector 501 is moved by the movement of theobjective lens, from an increase or a decrease in the amount of receivedlight in each divided region of the dual-divided optical detector 501, atracking error signal TR according to the ordinary push-pull method isrepresented by Equation (8), using parameters α (0<α<1), β (0<β<1)attributed to the movement of the objective lens: $\begin{matrix}\begin{matrix}{{{TR}\left( {\alpha,\beta} \right)} = {S\left( {I_{0,{+ 1}} + {\alpha\quad I_{0,{- 1}}} + {\beta\quad I_{0}} - {\left( {1 - \alpha} \right)I_{0,{- 1}}} - {\left( {1 - \beta} \right)I_{0}}} \right)}} \\{= {{{TR}\left( {0,0} \right)} + {2{S\left( {{\alpha\quad I_{0,{- 1}}} + {\beta\quad I_{0}}} \right)}}}}\end{matrix} & (8)\end{matrix}$

The second term in the right hand side corresponds to the off-set. Here,in Equation (5), if, for example, the sub-spots by means of thediffraction grating are shifted by one-half of the track pitch, a phaseinside cos in the third term in the right hand side is shifted by π.Accordingly, at this time, I′_(0, ±1) (x, y), i.e. interferenceintensities of the sub-spots, are represented with reference to u_(o),i.e. the spot position of the main spot, by Equation (9):$\begin{matrix}{{I_{0,{\pm 1}}^{\prime}\left( {x,y} \right)} = {{R_{0}}^{2} + {R_{\pm 1}}^{2} - {2{R_{0}}{R_{\pm 1}}{\cos\left( {\varphi \mp {\frac{2\pi}{P}u_{0}}} \right)}}}} & (9)\end{matrix}$

Moreover, in Equation (7), too, if the sub-spots are shifted by one-halfof the track pitch, the tracking error signal is inverted, too.Accordingly, TR′ (α, β), i.e. a tracking error signal of the sub-spotsat the time when there exists the movement of the lens, is representedby Equation (10):TR′(α, β)=−TR(0,0)+2S(αI′ _(0,−1) +βI ₀)  (10)

Consequently, by subtracting the tracking error signal of the sub-spotsfrom the tracking error signal of the main-spot, a signal expressed byEquation (11) is obtained: $\begin{matrix}\begin{matrix}{{{{TR}\left( {\alpha,\beta} \right)} - {{TR}^{\prime}\left( {\alpha,\beta} \right)}} = {{2{TR}\left( {0,0} \right)} + {2S\quad{\alpha\left( {I_{0,{- 1}} - I_{0,{- 1}}^{\prime}} \right)}}}} \\{= {{2{TR}\left( {0,0} \right)} +}} \\{4S\quad\alpha{R_{0}}{R_{\pm 1}}{\cos\left( {\varphi + {\frac{2\pi}{P}u_{0}}} \right)}}\end{matrix} & (11)\end{matrix}$

Accordingly, when an off-track of the main-spot is equal to 0, namely,u_(o)=0, the off-set is represented by the following Equation (12):$\begin{matrix}\begin{matrix}{{Offset} = {4S\quad\alpha{R_{0}}{R_{\pm 1}}\cos\quad\varphi}} \\{= {4S\quad\alpha\frac{w}{P^{2}}\sin\quad c\frac{w}{P}\left( {{2\frac{w}{P}} - 1} \right)\left( {1 - {\cos\quad 4\quad\pi\quad d}} \right)}}\end{matrix} & (12)\end{matrix}$

Consequently, if the width of the groove is not one-half of the trackpitch, the off-set remains. This is caused by the fact that, as seenfrom the second term in the right hand side in the upper stage ofEquation (11), when the main-spot is on-track, the interferenceintensity thereof differs from interference intensities of thesub-spots. Thus, by anticipating this intensity variation and setting inadvance a gain G₂ shown in FIG. 4, it is possible to cancel the off-set.Also, in the case of an optical disk such as the DVD-RAM in which theland-groove type optical disk is employed, the off-set is canceled outautomatically without setting such a gain.

The above-described description has been given concerning the effect ofcanceling the off-set in the tracking error signal. The method employedtherefor is that a light beam the interference phase of which isinverted is detected simultaneously. By the way, this method alsocancels out the disturbance into the focus error signal when an opticalspot crosses the guiding groove, which appears as a serious problem inthe astigmatic focal point shift detecting method. The principle thereofwill be explained below: First of all, there exist two major causesconcerning the above-mentioned disturbance in association with thecrossing over the track in the astigmatic focal point shift detection.One is astigmatism exerted upon the optical spot on the disk. The otheris a shift in the four-divided optical detector. Here, the explanationwill be given by employing, as the example, a mixture of the disturbancecaused by the astigmatism. Using W₂₂, i.e. an aberration coefficient ofastigmatism, and Φ, i.e. direction angle of astigmatism, a wave surfacehaving the astigmatism is represented by Equation (13):W(ρ, θ)=W ₂₂ρ² cos2(θ−φ)  (13)

-   ρ: Normalized radial coordinate in the pupil-   W₂₂: Aberration coefficient of astigmatism-   θ: Polar angle coordinate in the pupil-   φ: Direction angle of astigmatism

This can be rewritten into the Equation (14), using x, y coordinates ofan effective diameter in the pupil:

 W(x, y)=W ₂₂{(x ² −y ²)cos 2φ+2 xy sin2φ}  (14)

-   -   x,y: Normalized Cartesian (effective diameter) coordinates in        the pupil

Accordingly, assuming that the wave surface having the astigmatism isdiffracted by the optical disk and the 0th order light and the ±1storder diffracted lights thereof are shifted by ±δ and then areoverlapped with each other on the objective lens pupil surface, a phasedifference in the interference added by the astigmatism can beapproximated as a form of Equation (15): $\begin{matrix}\begin{matrix}{{\Delta\quad W} = {{W\left( {{x \pm \delta},y} \right)} - {W\left( {x,y} \right)}}} \\{\cong {{\pm \frac{\partial W}{\partial x}}\delta}} \\{= {{\pm 2}{W_{22}\left( {{x\quad\cos\quad 2\phi} + {y\quad\sin\quad 2\phi}} \right)}\delta}}\end{matrix} & (15)\end{matrix}$

Then, using this equation, interference intensities between the 0thorder light and the ±1st order diffracted lights are represented byEquation (16): $\begin{matrix}{{I_{0,{\pm 1}}\left( {x,y} \right)} = {{R_{0}}^{2} + {R_{\pm 1}}^{2} + {2{R_{0}}{R_{\pm 1}}{\cos\left( {{\Delta\quad W} + {\varphi \mp {\frac{2\pi}{P}u_{0}}}} \right)}}}} & (16)\end{matrix}$

Here, as illustrated in FIG. 6, if representative points A, B, C, D arepicked up in a reflected light beam 602, which has astigmatism and isreflected from the optical disk, interference intensities at thesepoints are represented by Equations (17) to (20), using Equation (16):$\begin{matrix}{I_{A} = {C + {\alpha\quad\cos\left\langle {{W_{22}{\delta\left( {{\cos\quad 2\phi} + {\sin\quad 2\phi}} \right)}} + \varphi - {\frac{2\pi}{P}u_{0}}} \right\rangle}}} & (17) \\{I_{B} = {C + {\alpha\quad\cos\left\langle {{W_{22}{\delta\left( {{\cos\quad 2\phi} - {\sin\quad 2\phi}} \right)}} + \varphi + {\frac{2\pi}{P}u_{0}}} \right\rangle}}} & (18) \\{I_{C} = {C + {\alpha\quad\cos\left\langle {{W_{22}{\delta\left( {{\cos\quad 2\phi} + {\sin\quad 2\phi}} \right)}} + \varphi + {\frac{2\pi}{P}u_{0}}} \right\rangle}}} & (19) \\{I_{D} = {C + {\alpha\quad\cos\left\langle {{W_{22}{\delta\left( {{\cos\quad 2\phi} - {\sin\quad 2\phi}} \right)}} + \varphi - {\frac{2\pi}{P}u_{0}}} \right\rangle}}} & (20)\end{matrix}$

Assuming that, basically, these intensities appear without being variedon the detectors for detecting the focus error, an disturbance which, asshown in Equation (21), $\begin{matrix}\begin{matrix}{{AF} = {\left( {I_{A} + I_{C}} \right) - \left( {I_{B} + I_{D}} \right)}} \\{= {{- 2}\alpha\quad{\sin\left( {{W_{22}\delta\quad\cos\quad 2\phi} + \varphi} \right)}{\sin\left( {W_{22}\delta\quad\sin\quad 2\phi} \right)}{\cos\left( {\frac{2\pi}{P}u_{0}} \right)}}}\end{matrix} & (21)\end{matrix}$is cos waveform-like in shape with reference to the off-track u_(o) ismixed into the focus error signal. Here, a focused spot, in whichvariations in intensity distribution of a reflected light beam reflectedwhen the focused spot crosses the guiding groove are inverted, isgenerated simultaneously and is added to the focus error signal. Thistransaction eventually means that a quantity, which is obtained byshifting phase φ by π and thus by inverting a sign of the first sin inEquation (21), is added, and accordingly the disturbance is canceledout.

The difference in reflectance between the guiding groove and the guidinginter-groove has required the adjustment of the gain with the use of,for example, the width of the guiding groove. For instance, theabove-described adjustment has become necessary for the canceling of thetracking error signal off-set in the differential push-pull method.However, in the canceling of the disturbance mixed into the focus errorsignal, the gain adjustment is unnecessary.

FIG. 7 shows another embodiment for simultaneously detecting the lightbeam in which the polarities of intensity distribution variations of thereflected light beam reflected when the focused spot on the optical diskcrosses the guiding groove are inverted. In this embodiment, a lineardiffraction grating 701, which is located in parallel to a radialdirection of the optical disk, is employed. Consequently, it turns outthat the ±1st order diffracted lights formed by the diffraction gratingon the optical disk are located on the same track as the 0th orderlight. Also, consequently, three four-divided optical detection regions112, 113, 114 constituting an optical detector 702 for detecting thereflected light beam are located in parallel to a tangential directionof the optical disk.

Next, using FIG. 8, the description will be given concerning a detailedstructure of the diffraction grating 701 employed in the presentembodiment. The diffraction grating is constituted so that, asillustrated in FIG. 8, phase of the gratings is inverted with a periodof Dλ/(2NA·P) with reference to P, i.e. a period of the guiding grooves,NA, i.e. numerical aperture of the objective lens, and D, i.e. aneffective light beam diameter for a diffraction grating-insertedposition. This period is equal to an interval which is determined byshifts of reflected light beams of ±1st order diffracted lights 802, 803toward a 0th order light 801 of a diffracted light formed by the guidinggrooves of the optical disk. In a diffracted light formed by this kindof diffraction grating, phase of a wave surface of the diffracted lightis shifted by an amount of π for each period. Remembering that adiffraction grating is, originally, a hologram, this phenomenon can beunderstood easily. The hologram is produced by performing, on a filmsuch as a photographic dry plate, exposure and development processingsof an interference fringe formed by two high coherent lights such aslaser lights. When the hologram is irradiated with one of the lights atthe time of performing the exposure processing thereto, the other lightis reproduced as a diffracted light by means of the hologram. Then, asdescribed above, if the interference fringe is formed by causing aninterference to occur between the light the wave surface of which isshifted periodically by one-half of the wavelength and the light thewave surface of which is flat, it is quite natural that the interferencefringe should reflect the phase shift and discontinuously form a step ofone-half of the fringe. Accordingly, if, conversely, the light the wavesurface of which is flat is launched into such a diffraction grating, itturns out that wave surface of the diffracted light is shiftedperiodically by one-half of the wavelength.

FIG. 9 is a diagram for explaining a manner in which, when a diffractedlight formed by the phase-inverted diffraction grating is furtherdiffracted by the guiding grooves of the optical disk, phase shiftregions of the resultant diffracted light are overlapped. The diffractedlight by means of the phase-inverted diffraction grating is furtherdiffracted by the guiding grooves of the optical disk, and the 0th orderlight and the ±1st order diffracted lights are overlapped with eachother. However, between the diffracted lights which are adjacent to eachother, such as the 0th order light and the ±1st order diffracted lights,the phase shift regions are in contact with each other without beingoverlapped. FIG. 10 summarizes phase differences added by thephase-inverted diffraction grating at this time between any two ofdiffraction orders included in each of the regions indicated by a, b, c,. . . in FIG. 9. FIG. 10 shows that phase differences between the lightswhich have adjacent diffraction orders and make a contribution to thetracking error signal, such as the 0th order light and the ±1st orderdiffracted lights, are equal to π without exception. Moreover, phasedifferences between the lights the difference in the diffraction ordersof which is equal to 2, such as the 0th order light and the ±2nd orderdiffracted lights, are equal to 0. Accordingly, concerning the phasedifferences in the interference shown in the Equation (5), withoutcausing the sub-spots to be off-track by one-half of the track pitch, itis possible to embody inversion of the interference intensities which isequivalent thereto. This transaction, even if storage marks existasymmetrically on the both sides of the central spot, brings about noasymmetry in an amount of reflected light of the sub-spots.Consequently, it becomes possible to further stabilize the effect ofcanceling the off-set in the tracking error signal or the effect ofcanceling the disturbance into the focus error signal.

Here, the phase-inverted diffraction grating is not integrated with theobjective lens. Accordingly, it turns out that, when the objective lensmoves following the decentering of the optical disk, an optical axis ofthe phase-inverted diffraction grating and that of the objective lensare relatively shifted with each other. FIG. 11, which indicates thephase shift regions in this case, shows that the movement of theobjective lens, even if it occurs, results in a mere movement ofconnected portions between the phase shift regions, thus bringing aboutno obstacle to the inversion of the interference intensities.

FIG. 12 shows still another embodiment for simultaneously detecting thelight beam in which the polarities of intensity distribution variationsof the reflected light beam reflected when the focused spot on theoptical disk crosses the guiding groove are inverted. Here, instead ofthe phase-inverted diffraction grating in FIG. 7, a polarizing phaseshifter 1201 is employed. The polarizing phase shifter 1201 inverts aphase of only a linearly polarized light component, which is polarizedin a specific direction and launched into the polarizing phase shifter,in regions of a period of λD/(2NA·P). Then, the linearly polarized lightcomponent is split and detected just in front of an optical detector702, using a 3-beam Wollaston prism 1202. At this time, unlike the caseof the phase-inverted diffraction grating, there occurs no sub-spots,and thus there exists only one optical spot on an optical disk 107. Thiscondition makes it possible to reduce a loss in the amount of lightcaused by the sub-spots, thus being able to constitute an optical headsuitable for the writable optical disks.

FIG. 13 is a diagram for explaining the principle of the polarizingphase shifter. Here, an example using lithium niobate (LiNbO₃) ispresented. A lithium niobate substrate 1301 has a principal axis 1302having a refractive index anisotropy in the direction indicated in FIG.13. Then, proton exchanged regions 1303 are formed in the substrate inaccordance with grating patterns. Moreover, in accordance with thegrating patterns, dielectric material layers 1304 are formed. At thistime, a phase difference φ_(o) between ordinary rays 1305, 1306 whichare each launched into the grating patterns and therebetween, and aphase difference φ_(e) between extraordinary rays 1307, 1308 which areeach launched into the grating patterns and therebetween, arerepresented as the following equations, respectively: $\begin{matrix}{{\phi_{o} = {\frac{2\pi}{\lambda}\left\{ {{\left( {n_{d} - 1} \right)T_{d}} + {\Delta\quad n_{o}T_{p}}} \right\}}}{\phi_{e} = {\frac{2\pi}{\lambda}\left\{ {{\left( {n_{d} - 1} \right)T_{d}} + {\Delta\quad n_{e}T_{p}}} \right\}}}} & (22)\end{matrix}$

-   -   λ: Wavelength    -   n_(d): Refractive index of dielectric material layer    -   T_(d): Thickness of dielectric material layer    -   Δn_(o): Change of the ordinary refractive index of lithium        niobate by proton exchange (=−0.04)    -   Δn_(e): Change of the extraordinary refractive index of lithium        niobate by proton exchange (=0.12)    -   T_(p): Depth of proton exchanged region

Here, with the diffraction efficiency taken into consideration, settingthe respective phase differences to be appropriate design values andsolving Equation (22) as simultaneous linear equations with T_(d), i.e.a thickness of the dielectric material layers, and T_(p), i.e. a depthof the proton exchanged regions, as the unknowns, the solutions arerepresented by Equation (23): $\begin{matrix}{{T_{p} = {\frac{\lambda}{2\pi}\frac{\phi_{o} - \phi_{e}}{{\Delta\quad n_{o}} - {\Delta\quad n_{e}}}}}{T_{d} = {\frac{\lambda}{2\pi}\frac{{\Delta\quad n_{o}\phi_{e}} - {\Delta\quad n_{e}\phi_{o}}}{\left( {n_{d} - 1} \right)\left( {{\Delta\quad n_{o}} - {\Delta\quad n_{e}}} \right)}}}} & (23)\end{matrix}$which means that it is possible to design a polarizing grating whichallows desirable phase differences to be created concerning the ordinaryrays and the extraordinary rays independently of each other. Forexample, if the desirable result is that: the wavelength λ=0.66 μm, therefractive index of the dielectric material layers n_(d)=2.2, φ_(o)=0°,and φ_(e)=+180°, it will do to let T_(p)=2.06 μm and T_(d)=0.07 μm.Taking the values in this way makes it possible to selectively shift thephase concerning the proton exchanged regions by an amount of π, thusbeing able to expect the same offset canceling effect as that in theabove-described embodiments.

The description will be given below concerning a canceling effectobtained by a scalar diffraction simulation toward an disturbance in afocus error detection signal when a focused spot crosses the guidinggroove and a canceling effect obtained by the scalar diffractionsimulation toward an off-set which accompanies the movement of theobjective lens. FIG. 14 shows a focus error signal at the time whenthere exist the astigmatism, a spherical aberration and detectordeviations in a complex state in the ordinary focus error detectingsystem. The central portion is swelled, which demonstrates that thereoccurs a considerably large disturbance. On the other hand, FIG. 15shows a calculation result on the assumption that the phase-inverteddiffraction grating is used. This demonstrates that almost all of thedisturbance is canceled out. FIG. 16 shows a focus error signal obtainedby adding focus error signals of the main-spot and of the sub-spotsaccording to the differential push-pull method. This demonstrates thatalmost all of the disturbance can be canceled out.

FIG. 17 shows a case in which, with the objective lens being moved andin the ordinary astigmatic focus error detecting method, a trackingerror signal on the light-receiving surface is calculated. Thisdemonstrates that there occurs a considerably large off-set. Meanwhile,FIG. 18 shows a case in which the same calculation is performed usingthe phase-inverted diffraction grating. This demonstrates that almostall of the off-set is canceled out. Furthermore, FIG. 19 shows anembodiment in which the same calculation is applied in the differentialpush-pull method. This demonstrates that the off-set is made extremelysmall.

FIG. 20 is a diagram for showing an embodiment of an optical systemconstitution according to the present invention in which thereproduction is possible especially in a CD, a CD-R, a DVD-ROM, and aDVD-RAM. Two semiconductor lasers, i.e. a 650 nm semiconductor laser2001 for the DVDs and a 780 nm semiconductor laser 2002 for the CD andthe CD-R, are mounted. In view of spectroscopic characteristics ofreflectance of a CD-R storage film, the 780 nm semiconductor laser 2002is absolutely necessary for the reproduction in the CD-R. The respectivelights are launched into diffraction gratings 2003, 2004, respectively,thus generating the ±1st order diffracted lights. Here, the diffractiongrating 2003 for 650 nm wavelength is a diffraction grating described upto now in the present invention, and the diffraction grating 2004 for780 nm wavelength is a diffraction grating for forming sub-spots for a3-beam tracking method usually employed to detect a tracking in the CD.Then, the light with 650 nm wavelength is reflected at a dichromaticmirror 2005, passes through a beam splitter 2006, is reflected at atriangle reflection mirror 2007, and is converged on the DVD 2009 by aDVD/CD compatible objective lens 2008. Meanwhile, the light with 780 nmwavelength is reflected at the beam splitter 2006 and at the trianglereflection mirror 2007, and is converged on the CD or the CD-R disk 2009by the DVD/CD compatible objective lens 2008. Respective reflectedlights, by way of the DVD/CD compatible objective lens 2008 and thetriangle reflection mirror 2007, passes through the beam splitter 2006,the dichromatic mirror 2005 and an optical component G, then beingconverged on an optical detector 2010.

FIG. 21 is a diagram for explaining the optical component G, alight-receiving pattern constitution of the optical detector, and signalcalculation methods in a plurality of embodiments obtained by changingthe focal point shift detecting method in the above-mentioned opticalsystem constitution. When a beam size detection method is employed asthe focal point shift detecting method, a curvilinear diffractiongrating 2101 is employed as the optical component G. The curvilineardiffraction grating 2101, for each of a 0th order light and ±1st orderdiffracted lights generated by the diffraction grating 2003 or 2004,outputs optical spots to be situated somewhat before a focal point onthe optical detector surface and optical spots to be situated somewhatbehind the focal point thereon as ±1st order diffracted lights generatedby the curvilinear diffraction grating 2101. At this time, diffractionefficiency of the curvilinear diffraction grating 2101 is made largeenough. This prevents the 0th order light from being generated, thusmaking it possible to decrease the number of the detection regions. Inthis way, out of the six optical spots in total, from a set of the ±1storder diffracted lights generated by the curvilinear diffraction grating2101, a focal point shift error signal according to the beam sizedetection method is obtained. One of the 0th order lights generated bythe diffraction gratings 2003 and 2004 is received by a four-dividedoptical detector, thereby being able to obtain a DPD signal(differential phase detection) employed in the DVD-ROM. Also, it ispossible to obtain a push-pull signal in which the off-set is canceledout from one of the 0th order light and the ±1st order diffracted lightsgenerated by the diffraction grating 2003. It is possible to detect the3-beam tracking error signal for the CD from a difference in the amountof light between the ±1st order diffracted lights generated by thediffraction grating 2004. Also, it is possible to obtain a reproduced RFsignal from a total amount of light of the 0th order lights generated bythe diffraction gratings 2003 and 2004.

When a double knife edge method is employed as the focal point shiftdetecting method, a light dividing prism 2102 is employed as the opticalcomponent G. The light dividing prism 2102 divides each of thediffracted lights, which are generated by the diffraction grating 2003or 2004, into four lights on the optical detector surface. From the fourlights of one of the diffracted lights, a focal point shift error signalaccording to the double knife edge method is obtained. The signals, suchas the tracking error signal according to the push-pull method, the DPDsignal, the tracking error signal according to the 3-beam method, andthe RF signal, can be obtained, as shown in FIG. 21, in almost the sameway as in the beam size detection method.

In these focal point shift detecting methods, selection of the directionof the divided lines in the optical detector makes it possible tocomparatively suppress, in the deviations and the aberrations, too, theoccurrence of the disturbance which accompanies the crossing over theguiding groove. Accordingly, in the present embodiment, the constitutionof adding the light the variations in intensity distribution of whichare inverted is not presented in particular. However, depending on theconstitution of the optical system, it may become necessary from theother requirements to provide a constitution of the divided lines inwhich the disturbance occurs easily. In that case, focus error signalsof lights the variations in intensity distribution of which are invertedare added to each other, thereby allowing the disturbance to be reducedin the focus error detecting methods other than the astigmatic focalpoint shift detecting method, too. Consequently, the present inventionalso makes possible an optical system constitution which,conventionally, could not be employed from the viewpoint of thedisturbance which accompanies the crossing over the guiding groove. Thischaracteristic allows a flexibility in the design to be increased.

When the astigmatic focal point shift detecting method is employed asthe focal point shift detecting method, the optical component G isunnecessary. The reason is that an astigmatism which occurs when thelights pass through the dichromatic mirror can be substituted for theastigmatism for the astigmatic focal point shift detection. This isbased on a principle that, when a focused light is launched into aparallel flat plate, an astigmatism occurs. Here, in the focus errordetection, as described up to now from the viewpoint of the disturbance,the focus error signals of the 0th order light and the ±1st orderdiffracted lights are added to each other. Conventionally, when theastigmatism is introduced using the parallel flat plate, the parallelflat plate was inserted in such a manner as to form an angle of 45degrees toward the tracks so that the disturbance which accompanies thecrossing over the guiding groove does not occur easily. This kind ofrestriction, however, becomes unnecessary because of the canceling ofthe disturbance based on the present invention. Accordingly, in somecases, the present invention can be effective in making the optical headcompact in the whole size. Also, concerning the tracking error signalaccording to the push-pull method, the tracking error signal thedistribution of which is inverted is similarly subtracted. The othertransactions are performed in much the same way as in the cases in whichthe other focus error detecting methods are employed.

According to the present invention, by adding an inexpensive componentsuch as a diffraction grating to a fixed optical system without mountingit on the objective lens actuator, it is possible to fundamentallyeliminate the disturbance which occurs in the focus error signal inassociation with the decentering of an optical disk when an optical spotcrosses a track on the surface of the storage film. At the same time, itis possible to fundamentally cancel the off-set which occurs in thetracking error signal in association with the movement of the lens.

1. An optical head comprising: a semiconductor laser; means forgenerating a plurality of reflected light beams from at least oneforward spot on an optical disk in which a light-converging opticalsystem converges light emitted from the semiconductor laser onto theoptical disk having a periodic structure in a radial direction of theoptical disk so as to form the at least one focused spot on the opticaldisk, the plurality of reflected light beams having polarities ofintensity distribution variations which are substantially inverted toeach other when the periodic structure crosses the at least one focusedspot on said disk; an optical detection system which splits theplurality of reflected light beams and detects the split reflected lightbeams; and an electrical circuit which provides a focus error signal ofthe at least one focused spot and a tracking error signal from theplurality of reflected light beams; and wherein the electrical circuitadds respective focus error signals of the plurality of reflected lightbeams so that variations in the focused error signals caused by theintensity distribution variations cancel each other out, to provide adifference signal between the focus error signals, amplifies respectivetracking error signals of the plurality of reflected light beams ofwhich the polarities are substantially inverted from each other with again proportional to a ratio of a total amount of a reciprocal of eachof the reflected light beams when one of the at least focused spot is onan information track of the optical disk, and then takes a differencebetween the respective amplified tracking error signals to provide atracking error signal for the optical head.
 2. An optical head accordingto claim 1, further comprising a beam splitting element which splits thereflected light beams reflected from the optical disk, from an opticalpath of the semiconductor laser to the optical disk, wherein the meansfor generating the plurality of reflected light beams having polaritieswhich are substantially inverted from each other includes a diffractiongrating disposed between the semiconductor laser and the beam splittingelement, the diffraction grating being arranged so that gratings of thediffraction grating are tilted relative to the radial direction of theoptical disk so that two focused spots of ±1st order diffracted light onthe optical disk produced by the diffraction grating are shifted bysubstantially one-half of a period of the periodic structure in oppositedirections in the radial direction of the optical disk relative to afocused spot of 0th order diffracted light on the optical disk producedby the diffracting grating.
 3. An optical head according to claim 1,wherein the periodic structure of the optical disk includes guidinggrooves, and a width of one of the guiding grooves is different from awidth between adjacent guiding grooves.